I'm in the middle of a building project (extension plus existing house reconfig & EWI), really wanted to do something properly eco but frankly it's been a fight against the whole industry - architects, builders, suppliers. I'll post the lessons learned here at some point...!

My contractor had originally proposed 80mm EWI using some type of fibreboard which he (or more accurately his subbie) has used on a similar project locally. I'd seen this being installed and the quality looks OK. Our existing walls are 9in solid brick and pretty sound.

I proposed 100mm Grey EPS and that's what is in the spec. My contractor is of the "I've been doing this 30yrs and we do it this way" variety. He says 1) we don't need 100mm and 2) is proposing 80mm all round (i.e. including the reveals). This sounds not right to me. See image - his proposal is Option A but mine is Option B.

Any advice or links to authoritative sources of window detailing would be appreciated!

To cut a long story short, the person we thought we were dealing with is the builder's son. When they rocked up on site it turned out the guy 'managing' the jobs and the finances is the father who runs the company.

All the details I have seen have thinner reveals (presumably to avoid a massively thick window frame at sides and head.

But thinner insulation is thinner insulation! If you are replacing the windows (unless the openings are tiny) why not simply get wider stiles (a job we're doing now will have 40mm extension pieces added to thew uPVC frame) and go round in a decent thickness? Thermal bridges are sometimes unavoidable, but if you have the choice...

More insulation is generally better - however you need to be able to open the windows, but the window are there for a reason (light and view) - the fact that there is a window there means that you have a big hole in the insulation value of the wall - in that case you should consider the effect that thick reveal insulation has on your sight lines and the light coming in.

In other words be careful not to box your window in too much and cause the window to be ineffective as a window. Of the two choices presented I would go with Option B.

If it is 'some sort of fibre-board', at 80mm, I would seriously question whether it will comply with Building Regs.

90mm of graphite EPS (with a lambda of 0.32W/mK) (EDIT - OOPS! 0.032). Thanks, MarkyP! just scrapes in under the required 0.3W/m2K.

In too much of a rush to find Lambda for Rockwool, but my supplier says:

''Rockwool has fantastic thermal insulating properties. 120mm of our dual density slabs when attached to the exterior of a property will take the u-value down to 0.3 which means the building will then conform to building regulations.''

...and the wood-fibre products tend to have lambda values of around 0.040 - 0.044, poss a little worse than Rockwool.

I agree with Nick, what they are proposing may not meet regs if the 80mm fibre board is the only insulation in the build up. What is the fibre board exactly? Best bang for a given thickness of typical EWI insulation options is graphite EPS (grey, not white) with an thermal conductivity value of around 0.032. Wood fibre and mineral wool boards were up to and beyond 0.040 if memory serves (I have done my own EWI).

Ask for their u-value calculation. if they cant tell you ask for the product they plan to use and get the lamda from the data sheet and work out the u-value using an online calculator (free). Like I said, I doubt it will be sufficient for u value which will meet regs.

building control may compromise in retrofit where it can be shown that achieving the u-value isn't viable.

I used 100mm graphite EPS for my EWI but already had a 50mm fully filled cavity so 150mm insulation overall. From memory I think Nick is right that 90mm enhanced EPS is the typical thickness used to achieve the min u-value over a 9inch wall. But this is the minimum, If you have sufficient eaves overhang, it makes sense to go for thicker EWI as the cost of the extra EPS isn't much, lots of the cost is in the installation and rendering.

Posted By: goodevansMore insulation is generally better - however you need to be able to open the windows, but the window are there for a reason (light and view) - the fact that there is a window there means that you have a big hole in the insulation value of the wall - in that case you should consider the effect that thick reveal insulation has on your sight lines and the light coming in.

The light and view issues are very real, although there are a couple of ways to mitigate the problem:

(1) Move the windows outboard to the line of the insulation. That will improve the thermal performance (i.e. reduce bridging) and also increase the amount of light entering the window.

(2) Rake the insulation back as it gets further away from the window. This will allow more light to enter the window and doesn't have much impact on the effective U-value.